Obviously it’s not as easy as I thought, lots of factors affect the results.
Instead of tackling it this way, or while we have to wait for better OSM material, you could consider to add an easier way to check the calculated route for such problems. Maybe an option to jump from town to town, add waypoints, jump to the next town, village etc.
Or jump to parts of the route where waypoints are closer then 200 meters to each other, something like that.
Anyhow, now when I know how it works I know I should go through the route and check things manually. For me that’s ok because my routes seldomly are longer than 400km.